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  <div class="section" id="module-hashlib">
<span id="hashlib-secure-hashes-and-message-digests"></span><h1>14.1. <a class="reference internal" href="#module-hashlib" title="hashlib: Secure hash and message digest algorithms."><tt class="xref py py-mod docutils literal"><span class="pre">hashlib</span></tt></a> &#8212; Secure hashes and message digests<a class="headerlink" href="#module-hashlib" title="Permalink to this headline">¶</a></h1>
<p class="versionadded">
<span class="versionmodified">New in version 2.5.</span></p>
<p id="index-0"><strong>Source code:</strong> <a class="reference external" href="http://hg.python.org/cpython/file/2.7/Lib/hashlib.py">Lib/hashlib.py</a></p>
<hr class="docutils" />
<p>This module implements a common interface to many different secure hash and
message digest algorithms.  Included are the FIPS secure hash algorithms SHA1,
SHA224, SHA256, SHA384, and SHA512 (defined in FIPS 180-2) as well as RSA&#8217;s MD5
algorithm (defined in Internet <span class="target" id="index-1"></span><a class="rfc reference external" href="http://tools.ietf.org/html/rfc1321.html"><strong>RFC 1321</strong></a>). The terms secure hash and message
digest are interchangeable.  Older algorithms were called message digests.  The
modern term is secure hash.</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">If you want the adler32 or crc32 hash functions, they are available in
the <a class="reference internal" href="zlib.html#module-zlib" title="zlib: Low-level interface to compression and decompression routines compatible with gzip."><tt class="xref py py-mod docutils literal"><span class="pre">zlib</span></tt></a> module.</p>
</div>
<div class="admonition warning">
<p class="first admonition-title">Warning</p>
<p class="last">Some algorithms have known hash collision weaknesses, refer to the &#8220;See
also&#8221; section at the end.</p>
</div>
<p>There is one constructor method named for each type of <em class="dfn">hash</em>.  All return
a hash object with the same simple interface. For example: use <tt class="xref py py-func docutils literal"><span class="pre">sha1()</span></tt> to
create a SHA1 hash object. You can now feed this object with arbitrary strings
using the <tt class="xref py py-meth docutils literal"><span class="pre">update()</span></tt> method.  At any point you can ask it for the
<em class="dfn">digest</em> of the concatenation of the strings fed to it so far using the
<tt class="xref py py-meth docutils literal"><span class="pre">digest()</span></tt> or <tt class="xref py py-meth docutils literal"><span class="pre">hexdigest()</span></tt> methods.</p>
<p id="index-2">Constructors for hash algorithms that are always present in this module are
<a class="reference internal" href="md5.html#module-md5" title="md5: RSA's MD5 message digest algorithm. (deprecated)"><tt class="xref py py-func docutils literal"><span class="pre">md5()</span></tt></a>, <tt class="xref py py-func docutils literal"><span class="pre">sha1()</span></tt>, <tt class="xref py py-func docutils literal"><span class="pre">sha224()</span></tt>, <tt class="xref py py-func docutils literal"><span class="pre">sha256()</span></tt>, <tt class="xref py py-func docutils literal"><span class="pre">sha384()</span></tt>, and
<tt class="xref py py-func docutils literal"><span class="pre">sha512()</span></tt>.  Additional algorithms may also be available depending upon the
OpenSSL library that Python uses on your platform.</p>
<p>For example, to obtain the digest of the string <tt class="docutils literal"><span class="pre">'Nobody</span> <span class="pre">inspects</span> <span class="pre">the</span> <span class="pre">spammish</span>
<span class="pre">repetition'</span></tt>:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">&gt;&gt;&gt; </span><span class="kn">import</span> <span class="nn">hashlib</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">m</span> <span class="o">=</span> <span class="n">hashlib</span><span class="o">.</span><span class="n">md5</span><span class="p">()</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">m</span><span class="o">.</span><span class="n">update</span><span class="p">(</span><span class="s">&quot;Nobody inspects&quot;</span><span class="p">)</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">m</span><span class="o">.</span><span class="n">update</span><span class="p">(</span><span class="s">&quot; the spammish repetition&quot;</span><span class="p">)</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">m</span><span class="o">.</span><span class="n">digest</span><span class="p">()</span>
<span class="go">&#39;\xbbd\x9c\x83\xdd\x1e\xa5\xc9\xd9\xde\xc9\xa1\x8d\xf0\xff\xe9&#39;</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">m</span><span class="o">.</span><span class="n">digest_size</span>
<span class="go">16</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">m</span><span class="o">.</span><span class="n">block_size</span>
<span class="go">64</span>
</pre></div>
</div>
<p>More condensed:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">&gt;&gt;&gt; </span><span class="n">hashlib</span><span class="o">.</span><span class="n">sha224</span><span class="p">(</span><span class="s">&quot;Nobody inspects the spammish repetition&quot;</span><span class="p">)</span><span class="o">.</span><span class="n">hexdigest</span><span class="p">()</span>
<span class="go">&#39;a4337bc45a8fc544c03f52dc550cd6e1e87021bc896588bd79e901e2&#39;</span>
</pre></div>
</div>
<p>A generic <a class="reference internal" href="new.html#module-new" title="new: Interface to the creation of runtime implementation objects. (deprecated)"><tt class="xref py py-func docutils literal"><span class="pre">new()</span></tt></a> constructor that takes the string name of the desired
algorithm as its first parameter also exists to allow access to the above listed
hashes as well as any other algorithms that your OpenSSL library may offer.  The
named constructors are much faster than <a class="reference internal" href="new.html#module-new" title="new: Interface to the creation of runtime implementation objects. (deprecated)"><tt class="xref py py-func docutils literal"><span class="pre">new()</span></tt></a> and should be preferred.</p>
<p>Using <a class="reference internal" href="new.html#module-new" title="new: Interface to the creation of runtime implementation objects. (deprecated)"><tt class="xref py py-func docutils literal"><span class="pre">new()</span></tt></a> with an algorithm provided by OpenSSL:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">&gt;&gt;&gt; </span><span class="n">h</span> <span class="o">=</span> <span class="n">hashlib</span><span class="o">.</span><span class="n">new</span><span class="p">(</span><span class="s">&#39;ripemd160&#39;</span><span class="p">)</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">h</span><span class="o">.</span><span class="n">update</span><span class="p">(</span><span class="s">&quot;Nobody inspects the spammish repetition&quot;</span><span class="p">)</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">h</span><span class="o">.</span><span class="n">hexdigest</span><span class="p">()</span>
<span class="go">&#39;cc4a5ce1b3df48aec5d22d1f16b894a0b894eccc&#39;</span>
</pre></div>
</div>
<p>This module provides the following constant attribute:</p>
<dl class="data">
<dt id="hashlib.hashlib.algorithms">
<tt class="descclassname">hashlib.</tt><tt class="descname">algorithms</tt><a class="headerlink" href="#hashlib.hashlib.algorithms" title="Permalink to this definition">¶</a></dt>
<dd><p>A tuple providing the names of the hash algorithms guaranteed to be
supported by this module.</p>
<p class="versionadded">
<span class="versionmodified">New in version 2.7.</span></p>
</dd></dl>

<p>The following values are provided as constant attributes of the hash objects
returned by the constructors:</p>
<dl class="data">
<dt id="hashlib.hash.digest_size">
<tt class="descclassname">hash.</tt><tt class="descname">digest_size</tt><a class="headerlink" href="#hashlib.hash.digest_size" title="Permalink to this definition">¶</a></dt>
<dd><p>The size of the resulting hash in bytes.</p>
</dd></dl>

<dl class="data">
<dt id="hashlib.hash.block_size">
<tt class="descclassname">hash.</tt><tt class="descname">block_size</tt><a class="headerlink" href="#hashlib.hash.block_size" title="Permalink to this definition">¶</a></dt>
<dd><p>The internal block size of the hash algorithm in bytes.</p>
</dd></dl>

<p>A hash object has the following methods:</p>
<dl class="method">
<dt id="hashlib.hash.update">
<tt class="descclassname">hash.</tt><tt class="descname">update</tt><big>(</big><em>arg</em><big>)</big><a class="headerlink" href="#hashlib.hash.update" title="Permalink to this definition">¶</a></dt>
<dd><p>Update the hash object with the string <em>arg</em>.  Repeated calls are equivalent to
a single call with the concatenation of all the arguments: <tt class="docutils literal"><span class="pre">m.update(a);</span>
<span class="pre">m.update(b)</span></tt> is equivalent to <tt class="docutils literal"><span class="pre">m.update(a+b)</span></tt>.</p>
<p class="versionchanged">
<span class="versionmodified">Changed in version 2.7: </span>The Python GIL is released to allow other threads to run while
hash updates on data larger than 2048 bytes is taking place when
using hash algorithms supplied by OpenSSL.</p>
</dd></dl>

<dl class="method">
<dt id="hashlib.hash.digest">
<tt class="descclassname">hash.</tt><tt class="descname">digest</tt><big>(</big><big>)</big><a class="headerlink" href="#hashlib.hash.digest" title="Permalink to this definition">¶</a></dt>
<dd><p>Return the digest of the strings passed to the <a class="reference internal" href="#hashlib.hash.update" title="hashlib.hash.update"><tt class="xref py py-meth docutils literal"><span class="pre">update()</span></tt></a> method so far.
This is a string of <a class="reference internal" href="#hashlib.hash.digest_size" title="hashlib.hash.digest_size"><tt class="xref py py-attr docutils literal"><span class="pre">digest_size</span></tt></a> bytes which may contain non-ASCII
characters, including null bytes.</p>
</dd></dl>

<dl class="method">
<dt id="hashlib.hash.hexdigest">
<tt class="descclassname">hash.</tt><tt class="descname">hexdigest</tt><big>(</big><big>)</big><a class="headerlink" href="#hashlib.hash.hexdigest" title="Permalink to this definition">¶</a></dt>
<dd><p>Like <a class="reference internal" href="#hashlib.hash.digest" title="hashlib.hash.digest"><tt class="xref py py-meth docutils literal"><span class="pre">digest()</span></tt></a> except the digest is returned as a string of double length,
containing only hexadecimal digits.  This may  be used to exchange the value
safely in email or other non-binary environments.</p>
</dd></dl>

<dl class="method">
<dt id="hashlib.hash.copy">
<tt class="descclassname">hash.</tt><tt class="descname">copy</tt><big>(</big><big>)</big><a class="headerlink" href="#hashlib.hash.copy" title="Permalink to this definition">¶</a></dt>
<dd><p>Return a copy (&#8220;clone&#8221;) of the hash object.  This can be used to efficiently
compute the digests of strings that share a common initial substring.</p>
</dd></dl>

<div class="section" id="key-derivation-function">
<h2>14.1.1. Key Derivation Function<a class="headerlink" href="#key-derivation-function" title="Permalink to this headline">¶</a></h2>
<p>Key derivation and key stretching algorithms are designed for secure password
hashing. Naive algorithms such as <tt class="docutils literal"><span class="pre">sha1(password)</span></tt> are not resistant against
brute-force attacks. A good password hashing function must be tunable, slow, and
include a <a class="reference external" href="https://en.wikipedia.org/wiki/Salt_%28cryptography%29">salt</a>.</p>
<dl class="function">
<dt id="hashlib.pbkdf2_hmac">
<tt class="descclassname">hashlib.</tt><tt class="descname">pbkdf2_hmac</tt><big>(</big><em>name</em>, <em>password</em>, <em>salt</em>, <em>rounds</em>, <em>dklen=None</em><big>)</big><a class="headerlink" href="#hashlib.pbkdf2_hmac" title="Permalink to this definition">¶</a></dt>
<dd><p>The function provides PKCS#5 password-based key derivation function 2. It
uses HMAC as pseudorandom function.</p>
<p>The string <em>name</em> is the desired name of the hash digest algorithm for
HMAC, e.g. &#8216;sha1&#8217; or &#8216;sha256&#8217;. <em>password</em> and <em>salt</em> are interpreted as
buffers of bytes. Applications and libraries should limit <em>password</em> to
a sensible value (e.g. 1024). <em>salt</em> should be about 16 or more bytes from
a proper source, e.g. <a class="reference internal" href="os.html#os.urandom" title="os.urandom"><tt class="xref py py-func docutils literal"><span class="pre">os.urandom()</span></tt></a>.</p>
<p>The number of <em>rounds</em> should be chosen based on the hash algorithm and
computing power. As of 2013, at least 100,000 rounds of SHA-256 is suggested.</p>
<p><em>dklen</em> is the length of the derived key. If <em>dklen</em> is <tt class="xref docutils literal"><span class="pre">None</span></tt> then the
digest size of the hash algorithm <em>name</em> is used, e.g. 64 for SHA-512.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">&gt;&gt;&gt; </span><span class="kn">import</span> <span class="nn">hashlib</span><span class="o">,</span> <span class="nn">binascii</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">dk</span> <span class="o">=</span> <span class="n">hashlib</span><span class="o">.</span><span class="n">pbkdf2_hmac</span><span class="p">(</span><span class="s">&#39;sha256&#39;</span><span class="p">,</span> <span class="n">b</span><span class="s">&#39;password&#39;</span><span class="p">,</span> <span class="n">b</span><span class="s">&#39;salt&#39;</span><span class="p">,</span> <span class="mi">100000</span><span class="p">)</span>
<span class="gp">&gt;&gt;&gt; </span><span class="n">binascii</span><span class="o">.</span><span class="n">hexlify</span><span class="p">(</span><span class="n">dk</span><span class="p">)</span>
<span class="go">b&#39;0394a2ede332c9a13eb82e9b24631604c31df978b4e2f0fbd2c549944f9d79a5&#39;</span>
</pre></div>
</div>
<p class="versionadded">
<span class="versionmodified">New in version 2.7.8.</span></p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">A fast implementation of <em>pbkdf2_hmac</em> is available with OpenSSL.  The
Python implementation uses an inline version of <a class="reference internal" href="hmac.html#module-hmac" title="hmac: Keyed-Hashing for Message Authentication (HMAC) implementation for Python."><tt class="xref py py-mod docutils literal"><span class="pre">hmac</span></tt></a>. It is about
three times slower and doesn&#8217;t release the GIL.</p>
</div>
</dd></dl>

<div class="admonition-see-also admonition seealso">
<p class="first admonition-title">See also</p>
<dl class="last docutils">
<dt>Module <a class="reference internal" href="hmac.html#module-hmac" title="hmac: Keyed-Hashing for Message Authentication (HMAC) implementation for Python."><tt class="xref py py-mod docutils literal"><span class="pre">hmac</span></tt></a></dt>
<dd>A module to generate message authentication codes using hashes.</dd>
<dt>Module <a class="reference internal" href="base64.html#module-base64" title="base64: RFC 3548: Base16, Base32, Base64 Data Encodings"><tt class="xref py py-mod docutils literal"><span class="pre">base64</span></tt></a></dt>
<dd>Another way to encode binary hashes for non-binary environments.</dd>
<dt><a class="reference external" href="http://csrc.nist.gov/publications/fips/fips180-2/fips180-2.pdf">http://csrc.nist.gov/publications/fips/fips180-2/fips180-2.pdf</a></dt>
<dd>The FIPS 180-2 publication on Secure Hash Algorithms.</dd>
<dt><a class="reference external" href="http://en.wikipedia.org/wiki/Cryptographic_hash_function#Cryptographic_hash_algorithms">http://en.wikipedia.org/wiki/Cryptographic_hash_function#Cryptographic_hash_algorithms</a></dt>
<dd>Wikipedia article with information on which algorithms have known issues and
what that means regarding their use.</dd>
</dl>
</div>
</div>
</div>


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  <h3><a href="../contents.html">Table Of Contents</a></h3>
  <ul>
<li><a class="reference internal" href="#">14.1. <tt class="docutils literal"><span class="pre">hashlib</span></tt> &#8212; Secure hashes and message digests</a><ul>
<li><a class="reference internal" href="#key-derivation-function">14.1.1. Key Derivation Function</a></li>
</ul>
</li>
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